1. Field of the Invention
This invention relates generally to threaded tubular joints or connections and, more specifically, to a tubular joint for connecting the male or pin end of a pipe member to the female or box end of a pipe member in which a tapered radius seal surface provides a pressure energized seal for the connection.
2. Description of the Prior Art
A variety of threaded connections are known in the prior art for joining tubular flow conduits in an end-to-end relationship to form a continuous flow path for transporting fluid. Typical examples of such flow conduits include casing, expandable casing, tubing, drill pipe and risers for oil, gas, water and waste disposal wells, and in horizontal and trenchless drilling applications. In the case of oil field casing and tubing, it is a common practice to use metal pipes of a definite length, with sections of pipe joined to form a string. The string of pipes effectively creates one lengthier pipe, intended to provide a means to reach the depth at which the reservoirs of gas or oil are found in order for extraction to the surface.
The pipe sections are secured together at their ends by an externally threaded connector, or “pin” that is threadedly received within an internally threaded connector or “box”. Typically, each pipe section will have a pin on one pipe end and a box at the opposite pipe end. Some pipe has an internally threaded coupling secured to one end of a double pin pipe section to produce the box. The individual pipe sections are frequently referred to as a “pipe joint”. Tubing and casing pipe joints are usually 30 ft. in length but can vary in length from 20 ft. to 40 ft. or longer.
The various pipe strings used in constructing a well are usually assembled on the floor of a drilling or workover rig. The pipe string is lengthened and lowered into the well as succeeding pipe joints are added to the string. During this assembly procedure, the pipe joint being added to the string is lowered, pin down or pin up, into an upwardly or downwardly facing box projecting from the drilling rig floor. This procedure is commonly referred to as “stabbing” the pin into the box. After being stabbed, the added pipe joint is rotated to engage the threads of the pin and box, securing the joint to the string. The process is basically reversed in or to disassemble the pipe string. Once free of the box, the removed joint is moved to a storage location.
Oil and gas wells are currently being drilled which extend for thousands of feet into the surrounding subterranean formations. The connections for strings of drill pipe, tubing or casing must be able to withstand the total weight of a string of pipe many thousands of feet long. Since the drill string must also be used for the purpose of drilling, the joints must be able to withstand high torque loads, as well. Additionally, wells may not be driven in exactly vertical fashion or even in straight line fashion. Horizontal oil and gas well drilling operations are common today. Other common horizontal drilling operations include the so-called “trenchless drilling” operations performed, for example, by municipalities for water and sewer line installations. This type of drilling places bending loads on the drill, casing or tubing strings at various places along the length thereof. In other operations, sections of tubing or casing are sometimes driven into place, resulting in compressive loads being exerted. Thus, in addition to withstanding tremendous tensile loads, the threaded connections in the pipe string must also be able to absorb considerable compression loads.
The pipe strings of the type under consideration must have joints that provide a seal against leakage between mating threaded members. This can be achieved by providing a traditional metal-to-metal seal, upon make-up and a number of such designs exist in the prior art. With these types of seal systems, it is important that the mating sealing sections be free of defects or damage because, unless substantial surface-to-surface contact is maintained, leakage will likely occur. Damage can occur, however, as during the assembling of the joints in that the components to be assembled are heavy and unwieldy. Thus, it is often too difficult to handle the pipe sections without damage, particularly as a new pipe section is being “stabbed” into a box or coupling. The result is that threads can be blemished and that sealing surfaces can be damaged.
Known prior art threaded connections used on oil field tubular goods often utilized a combination of specially designed “premium” threads and tapered (conical) sealing surfaces that engage to form a metal-to-metal seal to contain high pressures. The premium threads often generated radial interference as a means for retaining the make-up torque of the connection and also to provide a secure seal. In some designs, a resilient seal ring was also included.
In order to contain high pressures, the radial interference of the prior art metal-to-metal conical sealing surfaces must be significantly greater than that of the threads to generate sufficient bearing stress to overcome the effect of machining tolerances and taper on the height of the threads most adjacent the seal and the effects of excessive pipe compound trapped in the threaded area. The presence of excessive metal-to-metal contact bearing stress at the conical sealing surfaces of the prior art containment seals often resulted in make-up damage, i.e., galling with the result that the sealing mechanism did not function reliably. More importantly, conical seals have a desirable nominal axial makeup position. Non-shouldered premium connections have critical diametrical tolerances and makeup torque restrictions to guarantee an axial makeup position so the conical seal does not accumulate excess or deficient diametrical interferences.
Known prior art designs also exist that combine a conical metal-to-metal seal with a wedge or other “hook” thread design as a torque stop. These type designs typically required a shallower angle for the metal-to-metal seal because of lenient axial tolerances required to machine the threads. In order to generate enough radial interference in the metal-to-metal seal to effect an efficient sealing mechanism, a great deal of rotation was required between initial contact of the sealing regions to final make-up. The longer the metal-to-metal seals were in contact during such rotation, the greater the tendency for galling. On the other hand, if interference was decreased to lessen the amount of rotational contact, sufficient contact forces were not always present to effect a reliable seal at final make-up.
Thus, despite the improvements in thread forms and seal structures for such thread forms discussed above, a need continues to exist for a thread form and associated seal arrangement which is capable of coupling tubular pipe sections quickly and efficiently, which forms a secure seal, which provides a more versatile design than existing designs, and which is economical to produce.